Ultra-broadband Organic Terahertz Spectroscopy for Insulating and Semiconducting Materials

Terahertz time-domain spectroscopy (THz-TDS) based on THz waves in the range of 0.1-15 THz is highly useful technique for variety of insulating, semiconducting, conducting, and biological materials based on polymers and molecules. Many commercial THz-TDSs are based on inorganic THz generators and inorganic THz detectors, show a limitation of bandwidth in both THz wave generation and detection, e.g., < 3 THz for ZnTe generator and detector. In addition, inorganic THz crystals exhibit low optical-to-THz conversion efficiency because of their low optical nonlinearity. To overcome this limitation, organic electro-optic crystals are used as THz generators and THz detectors for THz-TDS. In this presentation, recent progress in ultrabroadband THz-TDS based on organic electro-optic crystals is discussed. In addition, to show a potential of ultrabroadband THz-TDS, characterizations of insulating and semiconducting materials are discussed. For achieving ultrabroad bandwidth and efficient optical-to-THz conversion, organic electro-optic crystals should exhibit large second-order macroscopic effective first hyperpolarizability and small absorption in THz frequency range. Highly efficient organic THz crystals were recently developed through various molecular engineering techniques such as nonlinear optical engineering and phonon mode engineering.